1. Field of the Invention
This invention relates to controls for electric motors and more particularly to an apparatus for controlling a 3-phase induction motor.
2. Information Disclosure Statement
With the increasing cost of electrical power, great emphasis has been placed on increasing the efficiency of devices which consume electrical power. An appreciable amount of electrical power is consumed by electrical motors in industrial applications. Due to ease of power transmission and to ease of maintenance of the electric motors, the AC induction motor is preferred over DC power and DC motors. Therefore, most industrial electrical motors are AC induction motors. Whereas a single phase induction motor may be used in relatively small sizes, the 3-phase induction motor is typical in the larger applications. It is well known that the torque supplied by an induction motor is proportional to the square of the voltage applied to the stator windings. The iron losses within the induction motor are also approximately proportional to the square of the applied voltage. Industrial applications of induction motors quite often involve uses where the rated torque is required only for short and infrequent intervals, and the motor is "idling" for appreciable intervals. The alternative is to stop and start the motor as required, but it is well known that starting current is exceptionally high due to the low back emf of the motor and frequent starting also tends to shorten motor life and cause transients in the power lines. The power factor varies tremendously with motor load: at rated load, the power factor of an induction motor is at a maximum, while the power factor can be quite low when the motor is operating without load or with a small load. During this low power factor condition, there can be considerable current flow even though little power is produced since the current and voltage are considerably out of phase. The extra current flow without any additional power requirement results in IIR losses in the motor and elsewhere throughout the system. Additional charges are often incurred from the power generating source for such additional current flow, even though real power is not consumed.
Some of the approaches which have been developed to increase the efficiency of the AC induction motor involve changes to the motor itself. As a result, a new generation of AC induction motors has been created which is considerably more efficient than the older motors. One such well known approach is known as the Wanlass motor which attempts to optimize the value of a capacitor in series with the motor winding in order to increase the efficiency of the induction motor. An alternative general approach is to either synthesize and control the AC power for each individual induction motor according to requirements, or to install a motor control apparatus between a standard power source and a standard AC induction motor. A well known controller of the latter type is the power factor controller developed by Frank J. Nola which has been the subject of U.S. Pat. Nos. 4,052,648, 4,266,177, and 4,404,511. The Nola controller measures the zero crossings of the voltage and the current and increases the voltage applied to the motor as the difference in zero crossings decreases. Although the Nola controller can perform quite well under load conditions which do not vary too rapidly, there is some tendency for the Nola controller to exhibit instability. When the controller is adjusted to decrease sensitivity so as to avoid instabilities such as very low frequency oscillations, then the Nola controller tends to be relatively slow to respond to changes in the motor power factor and/or line voltaqe.
Another approach in the design of a motor controller to be installed in series between the AC source and the induction motor has been developed by the Vectrol Division of Westinghouse Electric Corporation. The Vectrol unit, marketed as the "Startrol Power Miser," measures the back emf across the electronic switch in series in each supply line during the time that the electronic switch is open, and subtracts out the line voltage in order to determine the slip of the motor. Current drawn by the motor is sensed by a current transformer associated with one of the supply lines in the 3-phase system. A major problem with the Vectrol design, however, is that the motor controller unit is essentially different in each application in that the current transformer must be selected carefully and matched to the particular size of motor to be controlled.
Another motor controller which measures back emf across the electronic switches in series with each supply line and subtracts out line voltage to determine slip, as well as utilizing a current transformer to measure current drawn by the motor is the design shown in FIG. 2 of this application and discussed in detail hereinafter. Although this improved design permits compensation for variations in current transformers by selecting the value of a feedback resistor, the degree of compensation is not sufficient to meet all situations and the system also tends to exhibit some lack of sensitivity to decreases in line voltage.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the pertinent art.
Another object of this invention is to provide a motor control apparatus for a 3-phase induction motor having a control means for controlling current flow in each supply line, the control means responding to phase control means and driver means which are controlled by a measurement of back emf across each such control means and by measurement of current drawn by the motor, wherein the back emf measurement is corrected for variations in line voltage by a sensing circuit which includes a pair of reverse-connected diodes to equalize the response of the motor control apparatus to both increases and decreases in line voltage.
Another object of this invention is to provide a motor control apparatus with an adjustable voltage divider used in conjunction with comparator means for compensating for variations in current transformers used to measure current drawn by the induction motor.
Another object of this invention is to provide a motor control apparatus for a 3-phase induction motor wherein the motor control apparatus has increased filtering of a reference voltage signal representative of line voltage.
Another object of this invention is to provide a motor control apparatus for a 3-phase induction motor wherein the motor control apparatus has a starting control means for providing a soft start for the motor, and wherein a special discharge path is provided to protect the final comparator in the starting control means.
Other objects and a fuller understanding of this invention may be had by referring to the summary of the invention, the description and claims, taken in conjunction with the accompanying drawings.